Background: Guideline for practice is useful because it can be the standard for assessment and way to improve quality. We need to take account of expert opinion and consensus as well as scientific evidences to develope practice guideline because it should be practical. Delphi method has been developed to gather opinions from experts fairly. This study was designed to develop practice guideline and apply it to practices as a method to improve quality in primary medical care. Methods: Hypertension(in adults over 18 years old) was selected as a target problem. Self questionnaires about management of hypertensive patients were developed by a researcher with advice of 11 experts. The questionnaires were designed to response as a 5 pont scale. The results of previous questionary were given to respondents in second and third questionnaires. If needed, the questionnaires were changed on the previous responses. The items with medium greater than or equal to 4 point in 1st and 2nd responses were accepted in guideline. The items with medium lesser than or equal to 3 point were questioned again about whether they can be excluded or not. The criteria for assessment was made with reference to guideline and applied to 85 hypertensive patients of 9 family practitioners. At 3 months after practice guideline had been given to family practitioners, performance of same practitioners was assessed with 36 new hypertensive patients. Results: 23 professors in family medicine, 22 family practitioners and 6 cardiologists, responded among 50, 50, 15 respectively. Practice guideline with 33 items was developed as a result of 3 times questionaires. The difference of responses between professors in family medicine, family practitioners and cardiologists was not significant. Performance of practice was improved in diagnosis, history taking about 6 fields, laboratory examination and decision making about time of pharmaceutical prescription. It was not improved in physical examination, life style modification, method of pharmaceutical prescription, choice of initial antihypertensives and history taking about duration of disease and diet habit. It was decreased in history taking about psychosocial factors. The assessment scores were low in history taking, physical examination and life style modification before and after use of practice guideline. Conclusion: Practice guideline for hypertension could be developed by Delphi method. Performance of practice improved partially after use of guideline.